1. Field of the Invention
This invention relates to xerographic imagers using a light emitter array. In particular, this invention is directed to architectures, characteristics and methods of using a time delay and integration technique with shift registers and organic light emitting diode (OLED) printbars used in such xerographic light emitter arrays.
2. Technical Background
One of the fundamental design challenges for xerographic imaging is getting enough light to the photoreceptor at sufficient print speed while providing adequate service lifetime of the printbar. Rapid progress in OLEDs has produced devices which emit light levels greater than computer monitors (300 cd/m.sup.2) and fluorescent tubes (3000 cd/M.sup.2) in both white and in colors collectively spanning the visible spectrum.
Lifetime studies of OLEDs indicate that diode lifetime is determined to first order by the total charge passed through the OLED. Thus the OLEDs operate for short times at high brightness or for long times at low brightness. The lower end of the OLED brightness range is most stable, generally sustaining lifetimes of greater than 10,000 hours. The higher end of the OLED brightness range is less stable. For example, OLED devices operating at 1500 cd/m.sup.2 currently have sustainable lifetimes of only about 500 hours.
In a one-dimensional page-width array of such OLEDs there is not currently enough brightness to print at a reasonable speed with reasonable reliability for commercial uses. Table 1 outlines the technical data for a xerographic printer using a single row OLED printbar having OLED emitters operating at 1500 cd/M.sup.2. The printbar is illuminating a photoreceptor requiring about 7.5 ergs/cm.sup.2. Thus, the print speed of the single row devices is about 0.29 pages/min. Moderate print speeds are above five pages/min, and a more desirable print speed is about 30 pages/min. The brightness deficit determined by this rough calculation is about 100x, especially when considering that the print speed calculation for the single row page-width array of OLEDs leaves no room for dead time. Actual inorganic diode based printbars typically have a duty cycle well under 50%, in part to minimize blur in the process direction.
Furthermore, the calculated print-speed is the speed before degradation, where the lifetime for the devices is the time to 50% output decay.
TABLE 1 ______________________________________ TECHNICAL DATA FOR A CONVENTIONAL SINGLE ROW OLED PRINTBAR ______________________________________ Light Emitter Inputs Average Wavelength 590 nm Avg. Luminous 450 1 m/W Efficacy LED Brightness 1500 cd/m.sup.2 LED Current Density 25 mA/cm.sup.2 Display Voltage 20 Volts Number of Rows 1 Array Fill Factor 88% Optical Inputs Lens Transmittance 90% Lens Effective F# 4.765 Lens Efficiency 1.0% Photoreceptor Dose 7.5 erg/cm.sup.2 Page Property Inputs Document Exc. Time 0 sec Fast Scan 300 in.sup.-1 Resolution Slow Scan 300 in.sup.-1 Resolution Fast Scan Length 14 in Slow Scan Length 8.5 in Fractional Line 100% Time Outputs Surface Luminous 0.4712 1 m/cm.sup.2 Flux Surface Radiance 0.0010 W/cm.sup.2 Surface Radiance 10472.0 ergs/sec .multidot. cm.sup.2 Photoreceptor 103.778 ergs/sec .multidot. cm.sup.2 Irradiance Pixel Size 0.0085 cm Pixel Current 1.79 uA Array Emitting Area 0.26 cm.sup.2 Array Width 0.08 mm Array Emission 27.50 ergs/sec Array Current 6.623643 mA Array (Max) Power 0.13 Watts Power Efficiency 0.2094% Page Dose 5758.05 ergs Page Time 209.42 sec Line Time 82.12 msec Print Speed 0.287 pages/min Data Rate 0.051 MHz ______________________________________
The brightness deficit is too large to compensate simply by running the diodes harder. For example, operating the OLEDs even briefly at 15000 cd/m.sup.2 would require such a high bias that the OLEDs would quickly become inoperative Furthermore, doing so would only increase the print speed of the single row array to 3 pages/minutes. In addition, the total lifetime print volume of the xerographic imager (&lt;9,000 pages) is insufficient.
Commonly assigned U.S. patent application Ser. No. 08/785,233, filed concurrently herewith, entitled "Integrating Xerographic Light Emitter Array with Grey Scale," the disclosure of which is incorporated herein by reference in its entirety, discloses one approach for using OLEDs operated at modest light levels to expose a photoreceptor drum or belt. This is accomplished by staging rows of emitters in the slow scan direction and moving the object image in sychronization with the movement of a photoreceptor past the array in the slow scan direction. The entire printbar can be rewritten during each line time of the photoreceptor, which allows the exposure on any spot on the photoreceptor to be varied over a number of grey levels equal to the number of stages.
Commonly assigned U.S. patent application Ser. No. 08/785,231, to Fork, filed concurrently herewith, entitled "Self Replacing OLED Printbar," the disclosure of which is incorporated herein by reference in its entirety, proposes another way to extend the lifetime of an OLED- based printbar. This is accomplished by creating a plurality of OLED printbars on a substrate, having all printbars share common optics and selecting a working printbar element in the event of a printbar element failure. Thus, printing is accomplished from a single row of emitters which operate at a high brightness and current. When one row burns out or decays to a level insufficient for printing, that row is deactivated, a new row is activated and printing continues.